Sculpting brush - low poly with normal map?
You got a very interesting problem to solve. You could bake normal map from high poly,sculpted chunk of terrain to a texture,and then use this texture as a brush in texture painting mode to paint onto a terrain texture and save it as a normal map for the terrain. 

To to that, You have to have new image texture active in material of a UV mapped plane. Place this plane above high poly terrain, turn on Selected to active in Bake options panel, select first high poly terrain, and then the plane and hit Bake - that's all you have to in general do. But here is more detailed tutorial on the topic